In the context of particle therapy, of cancers in particular, a particle beam of e.g. protons or heavy ions, e.g. carbon ions, is generated in a suitable accelerator. The particle beam is guided in a beam guide in a treatment room, where it occurs via an emergence window. In a particular embodiment, the particle beam can be directed from an accelerator into different treatment rooms-alternately. In the irradiation room, a patient who is to undergo therapy will be positioned on a patient table and immobilized if applicable.
In order to achieve particularly good capacity utilization of the particle therapy apparatus, in particular of the accelerator, the particle beam is guided into different irradiation rooms consecutively in order to utilize the time before and after the irradiation of a patient, e.g. the time which is required for positioning the patient in an irradiation room, by irradiating another patient in another irradiation room.
The irradiation of the target region, usually a tumor, is normally done in layers. Depending on its energy, the particle beam reaches different depths in the tissue, such that the tissue can effectively be divided into slice-like sections or layers of identical penetration depth. The focused particle beam is then moved over the individual layers of the target region, this being known as “beam scanning”, such that a plurality of points within a layer are irradiated, said points being located on a raster grid, for example. By means of correctly selecting the beam intensity or the energies, it is also possible accurately to irradiate regions having a complex shape. The arrangement of the layers and points to be irradiated is selected such that the planned dose distribution can be achieved.
The measures that are described can be utilized in the context of various scanning methods.
In the context of the so-called spot scanning method, the particle beam is directed at each target point for a predetermined time and/or deposits a predetermined number of particles at each target point, and is switched off while deflection magnets etc. are adjusted to a next target point.
In the context of the so-called raster scanning method, the particle beam is directed at each target point for a predetermined time period or deposits a predetermined number of particles at each target point, but is not or not always switched off between the target points.
In the context of so-called continuous scanning methods, the target points form contiguous lines, i.e. continuous (or quasi-continuous) sets, their number being countably infinite. In the context of a continuous scanning method, the particle beam is continuously deflected at least within a line or row in an isoenergy layer, and scans the target points without lingering at individual locations. Using a depth modulation device, it is also possible to carry out a continuous scanning method in which the penetration depth of the particle beam is continuously modulated.
The movement of the beam and the adjustment of the beam energy are controlled by a control entity. The range of the particle beam is normally varied using an energy adjustment at the accelerator by means of the control entity, this being known as active energy variation. In this case, the active energy variation requires the magnetic strength to be adjusted to the energy of the beam, generally at all magnets or at least at many magnets in the subsequent high-energy beam transport path. In this case, alteration times occur in which no beam can be applied, even though the time is required for the purpose of irradiation. The energy alteration time between two successive energy levels or layers in the target region, i.e. the dead time during the energy alteration, is usually approximately 1-2 s. In the case of a plurality of energy alteration operations, the total dead time then amounts to several seconds or even to more than a minute.